Transport across the cell membrane

• charged and non-lipid soluble substances

• Integral proteins acting as pores, channels or carriers - allow these substances to cross the membrane

• Selectively permeable

Composition of the plasma membrane

• solubility in lipids

• Driving forces

• Molecular size

• Active or passive transport

• Transmembrane proteins - water-soluble substances

Permeability

• diffusion - through the lipid bilayer

• Diffusion - through ion channels - water soluble

• Facilitated diffusion - using a carrier - water soluble

Passive transport

• respiratory gases

• Lipids

• Small alcohols

• Urea

• Moves with the concentration gradient

Lipid substances

• ions

• Small sugars

• Amino acids

• Require integral membrane proteins

• Moves with concentration gradient

Water soluble substances

• a solute binds to a specific transporter on one side of the membrane is released to another

• Glucose and fructose

• Rate of movement depends on the steepness of the conc gradient and number of transporter proteins in membrane

Facilitated diffusion

• Ion channels

• Open and close based on electrochemical gradient

• Selective and specific

• Faster than facilitated diffusion

Gated protein channels

• works against concentration gradient

• Primary active transport - derived directly from ATP

• Secondary active transport - derived indirectly from ATP

Active transport

• metabolic (atp hydrolysis)

Primary active transport

• cotransport Na and H

Secondary active transport

• requires 40% of cellular ATP

• All cells have many of them

• Low Na and high K in cytosol

• Operates continuously

NA/K pump

• energy is stored in NA or H concentration gradients use to drive other substance against their own concentration gradients

• Two substances transporting in the same direction - symporters

• Two substances going in opposite directions - anti-porter

Secondary active transport

• movement of large molecules out of the cell

• Secretory cells

• Secretions in vesicles - neurotransmitters

Exocytosis

• movement into cell

Endocytosis

• engulfing small particles

Pinocytosis

• engulfing large particles

Phagocytosis

• movement of specific substances into the cell involving the caveolae regions of the cell membrane

Receptor medicated endocytosis